1. Field of the Invention
The present invention relates to a torque sensor, and more particularly to a torque sensor suitable to be applied to an electric power steering apparatus for an automobile.
2. Description of the Prior Art
An electric power steering apparatus has been developed that assists the power to operate a steering wheel of the automobile. It is constructed so as to detect a torque applied to the steering wheel and rotate an electric motor provided at a steering mechanism corresponding to the detected torque.
As the torque detection means, a torque detecting apparatus disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Sho 59-208431, is well-known. FIG. 1 is a partially cutaway perspective view of the torque detecting apparatus. The torque detecting apparatus comprises a shaft 40 to be measured which is of joint construction of two shafts 41 and 42 connected through a torsion bar 46 in spline-connection and engageable with each other through a predetermined gap d. Also, the apparatus has a tubular member 45 of magnetic material covering the outer periphery of the joint at the shaft to be measured 40 and fixed to both axes 41 and 42. In additin, the apparatus has an exciting coil (not shown) disposed outside the tubular member 45 to alternately magnetize the tubular member 45 in the predetermined direction. The apparatus also has a detecting coil (not shown) outputting the magneto striction component corresponding to a torque from the magnetic flux flowing in the tubular member 45.
When the torque applied to the shaft to be measured 40 is relatively small, the torque detecting apparatus applies a torsional moment to the tubular member 45 and a smaller diameter portion of torsion bar 46 by play existing between engaging pawls 43, 44 of the shafts 41, 42. Thereby a torsion amount changes with respect to a fine torque. On the other hand, when the torque is relatively larger, the engaging pawls 43 and 44 abut against each other and the torsional moment acts also on the tubular member 45, the smaller diameter portion of torsion bar 46, and shafts 41 and 42. Therefore, strength with respect to the torsional moment abruptly increases to reduce variation in the torsional amount of tubular member 45 with respect to variation in torque. A rate of change of the detection output of the magneto striction component for the torque is less than that when the engaging pawls 43 and 44 abut against each other, thereby enabling detection with high accuracy.
However, the torsional amount of the shaft to be measured 40, when the shaft to be measured 40 is subjected to the torque by steering, is extremely minute, whereby variation in the circumferentially opposite gap d between the engaging pawls 43 and 44 at the aforesaid torque detecting appparatus is a little. Variation in the magneto striction component flowing in the tubular member 45 corresponding to torque variation, that is, variation in the torque detection signal, is reduced. This created the problem in that sensitivity with respect to the torque variation acting on the shaft to be measured 40 is low and does not properly assist a steering force. Also in that the cylindrical tubular member 45 generates at the peripheral surface an eddy current to deteriorate the sensor output.
On the other hand, when a torque sensor is assembled while applying torsion to the torsion bar 46 during the assembly process, the circumferentially opposite gap d varies, thereby creating the problem in that a detection signal varies by the factor other than torque acting on the torsion bar 46.
Also variation in ambient temperature causes the opposite gap d to expand or contract, thereby creating the problem in that when the ambient temperature varies, the detection signal varies.